與隨機(jī)捕撈的銀河魚（中）相比，拿走大魚（下）和拿走小魚（上）都將對種群個(gè)體的平均大小產(chǎn)生深遠(yuǎn)的影響,。（圖片提供:Stephan Munch）
漁夫都愛釣大魚,，久而久之，這種做法會(huì)使魚類種群在遺傳層面上保持較小的個(gè)體,。然而一項(xiàng)新的研究表明,，這種向下的螺線是可以逆轉(zhuǎn)的,。如果漁夫能夠隨機(jī)地撒網(wǎng),，那么大魚的儲(chǔ)備就會(huì)出現(xiàn)一個(gè)反彈。
大多數(shù)水產(chǎn)業(yè)都只把一個(gè)種群中的大魚當(dāng)做目標(biāo),。拖網(wǎng)使得最小的魚能夠逃脫,，而只把釣魚當(dāng)做消遣的人們通常會(huì)把那些小家伙扔回大海。在這種情況下,，如今因大量捕撈而在上世紀(jì)80年代面臨滅頂之災(zāi)的加拿大鱈魚的個(gè)體要遠(yuǎn)遠(yuǎn)小于歷史記錄的水平,。這樣的趨勢讓漁業(yè)科學(xué)家們感到無比擔(dān)憂，因?yàn)榕c小魚相比,，大魚往往更健康并且繁殖力更強(qiáng),。
研究人員一直想搞清，他們在漁船甲板上看到的魚類個(gè)體平均尺寸的下降是否源于環(huán)境因素,，抑或魚類種群的遺傳變化,。2002年，美國紐約州立大學(xué)石溪分校的海洋生態(tài)學(xué)家David Conover和他的同事發(fā)現(xiàn),，遺傳因素是造成這一局面的部分原因,。在實(shí)驗(yàn)室分析中，Conover的研究小組在4年或4代的時(shí)間里,，從兩組銀河魚（Menidia menidia,，生活在美國東北海域的一種具有商業(yè)價(jià)值的魚類）中拿走了90%的大魚。結(jié)果顯示,，隨著體型大,、生長快的銀河魚從種群中消失，那些在遺傳上生長速度較慢的銀河魚逐漸在種群中占據(jù)了統(tǒng)治地位,，并導(dǎo)致銀河魚平均尺寸的下降,。
Conover于是便想搞清這一過程是否有逆轉(zhuǎn)的可能性,。研究人員這一次還是用縮小的銀河魚作為研究對象。他們從每組中拿走了90%的銀河魚,，但這一次研究人員并不是專揀大的撈,，而是隨機(jī)選擇。經(jīng)過6代的時(shí)間,，這些魚又重新增加了50%的體長,。研究小組在最近出版的英國《皇家學(xué)會(huì)學(xué)報(bào)B》上報(bào)告了這一研究成果。
據(jù)Conover分析,，這一結(jié)果意味著“如果我們停止捕撈那些已經(jīng)變得很小的魚類……那么就可以期待大魚的回歸”,。以銀河魚為例，他推算這種情況會(huì)在12年或12代之后發(fā)生,。Conover表示,，對包括大麻哈魚在內(nèi)的其他經(jīng)濟(jì)魚類而言，由于每一代的時(shí)間間隔更長,，因此實(shí)現(xiàn)同樣的過程大概需要幾十年,。
挪威卑爾根大學(xué)的進(jìn)化生態(tài)學(xué)家Christian Jorgensen指出：“這一積極的信息告訴我們逆轉(zhuǎn)是可能的，并且相當(dāng)快,。”然而芬蘭赫爾辛基大學(xué)的生物統(tǒng)計(jì)學(xué)家Anna Kuparinen卻指出,，在實(shí)際捕撈過程中，漁夫們并不能像在實(shí)驗(yàn)室中那樣高效地識別每條魚的大小,，因此種群所面臨的選擇壓力并沒有那么高,。Kuparinen表示，正是基于這種原因,，你很難確定一種野生種群是否會(huì)在捕撈過程中產(chǎn)生類似的變化,。（生物谷Bioon.com）
生物谷推薦原始出處：
Proc. R. Soc. B March 4, 2009, doi: 10.1098/rspb.2009.0003
Reversal of evolutionary downsizing caused by selective harvest of large fish
David O Conover1,*, Stephan B Munch1 and Stephen A Arnott1,2
1School of Marine and Atmospheric Sciences, Marine Sciences Research Center, Stony Brook University Stony Brook, New York, NY 11794-5000, USA
2South Carolina Department of Natural Resources, Marine Resources Research Institute PO Box 12559, Charleston, SC 29422, USA
Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution. We subjected six populations of silverside fish (Menidia menidia) to three forms of size-selective fishing for five generations, thereby generating twofold differences among populations in mean weight and yield (biomass) at harvest. This was followed by an additional five generations during which size-selective harvest was halted. We found that evolutionary changes were reversible. Populations evolving smaller body size when subjected to size-selective fishing displayed a slow but significant increase in size when fishing ceased. Neither phenotypic variance in size nor juvenile survival was reduced by the initial period of selective fishing, suggesting that sufficient genetic variation remained to allow recovery. By linear extrapolation, we predict full recovery in about 12 generations, although the rate of recovery may taper off near convergence. The recovery rate in any given wild population will also depend on other agents of selection determined by the specifics of life history and environment. By contrast, populations that in the first five generations evolved larger size and yield showed little evidence of reversal. These results show that populations have an intrinsic capacity to recover genetically from harmful evolutionary changes caused by fishing, even without extrinsic factors that reverse the selection gradient. However, harvested species typically have generation times of 3–7 years, so recovery may take decades. Hence, the need to account for evolution in managing fisheries remains.
